When the propagation environment where the sound pulse propagates is the underwater, for example, a sonar is widely used as a method for detecting targets existing in the underwater. That is, the distance and direction of a target are acquired through sweeping the underwater by using a sonic beam and receiving a reflected wave from the target that comes within the sonic beam. For extending the detection distance, it is necessary to use a low frequency sonic wave (e.g., 500 Hz) of a small absorption attenuation. However, when the frequency of the sonic wave is lowered, the sonic beam becomes widened and further increases an influence by reflections at the sea surface and the seabed, thereby making it difficult to detect the target.
Therefore, recently, there has been an increasing interest in a new target detection method using phase conjugated waves and time reversal waves.
C. Prada et al. perform so-called the time reversal processing which receives backward scattered waves scattered from a target existing in the water towards a sound source side and reverses the time axis of the received wave signal, and re-radiates the time reversal signal into the water. The re-radiated sonic wave converges to the position of the target. The converged sonic wave generates a reflection wave that is stronger than before. The reflected wave is received, and the time reversal processing is performed thereon again to be re-radiated. The sonic wave strongly converges to the position of the target.
Through repeating the above-described operation, it is possible to specify objects in order from larger ones to smaller ones.    Non-Patent Document 1: C. Prada, S. Manneville, D. Spoliansky, and M. Fink, “Docomposion of the time reversal operator: detection and selective focusing on two scatterers”, J. Acoust. Soc. Am. 99, 2067-2076 (1996)